The present invention relates to civil engineering and road construction machinery, and more particularly to a hydropneumatic percussive tool.
The invention can find application in compacting cohesive and loose soils of various physical and mechanical properties in conjested areas, breaking up concrete and asphalt concrete pavements or rock, shattering firm and frozen ground, and driving piles.
The invention may also be utilized in other industrial fields where it is necessary to provide considerable impact loads on working implements at relatively small overall dimensions of the source of such impact loads, for example, in the mining and metallurgical industries, as well as in mechanical engineering and public utilities.
As is known, percussive machines are generally classified into pneumatic or air powered (various jack, riveting, chopping hammers, concrete breakers, drills and soil compactors), electric (normally hand-operated machines with low impact power) and hydraulic (ranging from small hand-operated ones to large machines with impact energy of up to 10,000 j).
Inherent in hand-operated percussive machines of known construction are high levels of noise and vibration causing such an occupational hazard as vibrotrauma. Vibration also results in reduced power, efficiency and reliability of percussive machines which in turn affect the productivity and quality. Direct contact of the operator with such machines dictates some special safety features to be incorporated into their design in terms of reducing their weight, the level of vibration and eliminating the hazard of electric shock.
A study of techniques used to reduce recoil and vibration of percussive tools has shown that the modern trend is toward the provision of dynamically balanced machines of high impact frequency and power. A most optimum arrangement is the one where the piston hammer cooperates with an added mass or body without actually striking it, this body being stopped for effecting a return stroke with recuperation of energy. Such an arrangement assures reduced recoil, vibration and noise, along with improved performance compared with other known arrangements.
It is a common knowledge that pneumatic percussive tools, especially manually operated tools, are most noise and vibration hazardous. Electric percussive machines also feature high level of vibration along with heavy weight and low impact power.
An increasingly growing trend lately is toward making use of hydraulic power drives in various branches of the national economy, particularly in road construction machinery and civil engineering; this being caused by a number of advantages offered by hydraulic drives versus other power sources.
One promising direction taken by designers of new tools is to produce detachable mounted equipment well suited for the existing basic hydraulic machines.
This has given an impetus to create highly efficient, low-noise and low-vibration percussive machines powered by multi-purpose hydraulic drives.
At present, there are known numerous constructions of hydraulic and hydropneumatic percussive tools.
A hydropneumatic percussive tool is known (cf. USSR Inventor's Certificate No. 564,415, IPC E21C 3/20) comprising a housing having an implement and radial passages connected to pressure and discharge hydraulic lines. Arranged inside the housing is a reciprocating piston hammer of stepped configuration defining in conjunction with the housing three chambers, one of which communicates by way of passages with the pressure line, the second one connects to the discharge line, while the third chamber is filled with a compressed gas to functions as a pneumatic accumulator. The tool also comprises a hydraulic two-stage distributor controlled according to the position of the piston hammer having in the cylindrical interior thereof a control valve separating this interior into two portions, the hydraulic distributor being provided with radial and axial passages, one of the passages being fitted with a flow restrictor. By means of the passages of the distributor, one of the two portions of the cylindrical interior is connected to the chamber of the housing which communicates with the pressure line, whereas the other portion is connected with another chamber of the housing communicating with the discharge line, the output stage of the hydraulic distributor defining in conjunction with the stepped configuration of the housing two chambers of various diameters, the chamber of smaller diameter continuously communicating with a flow control distributor formed by two grooves made on the step of smaller diameter of the piston hammer and by two recesses in the step of the housing which contacts with the step of smaller diameter of the piston hammer.
The movement of the control valve to effect the idle and work strokes of the piston hammer is caused by the liquid supplied by a pump. However, the pump delivery rate is not fully utilized when the control valve moves in a position enabling the piston hammer to effect its work stroke, as part of the liquid delivered by the pump tends to escape through the flow restrictor arranged in the passage communicating the chamber of the hydraulic distributor connected to the discharge line whereby the movement of the control valve is slowed down or delayed resulting in a reduced frequency of impacts produced by the piston hammer and consequently in weakened impact power thereof. Another disadvantage of the above construction resides in complicated techniques employed for manufacturing the hydraulic distributor because the latter must be provided with a variety of grooves, axial and radial passages, as well as due to that the control valve must be provided with two mounting surfaces.
In addition, the recoil reaction acting on the housing and occuring during the work stroke of the piston hammer when the pressure of liquid in the housing chambers communicating with the pressure and discharge lines is equal, while the compressed gas occupying the pneumatic accumulator has an overpressure, is directed against the path of travel of the piston hammer. The maximum value of the recoil reaction is determined by a product of the overpressure of the compressed gas by the piston hammer area. In the return stroke the piston hammer moves in a direction opposite to the work stroke thereof; in consequence, the direction of the recoil reaction also changes. Therefore, the recoil reaction alternates.
Also known is a hydropneumatic percussive tool provided with a means for damping a recoil reaction (cf. USSR Inventor's Certificate No. 579,134, IPC B25D 9/12, E21C 3/22) comprising a housing having radial passages communicating with pressure and discharge hydraulic lines. Arranged inside the housing for axial reciprocations therein is a stepped piston hammer having steps of larger and smaller diameters separating the interior of the housing into three chambers, one of the chambers communicating by way of a passage with the pressure line, the second chamber connects to the discharge line, whereas the third one is defined by the step of larger diameter and the inner walls of the housing and is filled with a compressed gas to serve as a pneumatic accumulator. An inertia piston is provided arranged in the housing coaxially with the piston hammer and having steps of larger and smaller diameters equal to the corresponding diameters of the piston hammer, the inertia piston defining with the housing two chambers, one of which is connected by way of a passage to the pressure line, the other chamber defined by the housing and the step of larger diameter being filled with a compressed gas to serve as a pneumatic accumulator. The piston hammer is provided with a hydraulic distributor having radial and axial passages, a cylindrical chamber of the distributor accommodating a spring-loaded control valve separating this chamber into two portions one of which is continuously connected by way of passageways to the chamber communicating with the discharge line, the other one being connected with a groove made on one of the steps of larger diameter of the piston hammer. In addition, the hydraulic distributor is provided with a radial passage periodically closed by the control valve and continuously communicating with the chamber which is connected to the pressure line, the two pneumatic accumulators being separated by a rigid wall although communicating with each other, the pressure inside the pneumatic accumulator of the piston hammer being in excess of the pressure in the pneumatic accumulator of the inertia piston, the chambers communicating with the pressure line being connected therebetween by a pipe provided with a check valve.
When a liquid is delivered from a pump into the chambers connected to the pressure line, it acts on the piston hammer and the inertia piston which are caused to move toward each other thereby compressing the gas contained in the pneumatic accumulators (idle stroke of the piston hammer). At the end of the idle stroke the piston hammer closes by the step of larger diameter the passage of the housing which is connected to the discharge line to discommunicate therefrom the chamber normally connected therewith. The liquid in this chamber acts on the control valve which in turn begins to move and opens the passage of the hydraulic distributor connected to the chamber communicating with the pressure line. When this passage opens, the chambers connected to the pressure and discharge lines intercommunicate to equalize the pressure of liquid therein. Under the action of the compressed air in the two pneumatic accumulators the piston hammer and the inertia piston accelerate (work stroke of the piston hammer). At the end of the work stroke the piston hammer strikes against the implement, whereas the control valve induced by the spring acts to close the passage of the hydraulic distributor connected to the chamber which is communicating with the pressure line. Therewith, the inertia piston decelerates due to the liquid being forced out of the chamber defined by the inertia piston and the housing and communicating with the pressure line into the chamber formed by the piston hammer and the housing causing the piston hammer to move in the direction of the idle stroke and compressing the gas in the pneumatic accumulator. After the piston hammer and the inertia piston have stopped, the flow of liquid occupying the chamber formed by the piston hammer and the housing and connected to the pressure line is closed by the check valve. The liquid delivered by the pump causes the inertia piston to move in the direction of the idle stroke thereby compressing the gas in the pneumatic accumulator. When pressure in the two hydraulic accumulators equalizes, the check valve opens and the liquid is admitted to the two chambers connected to the pressure line, this being accompanied by a syncronous movement of the piston hammer and the inertia piston, whereupon the cycle is repeated.
The above device is disadvantageous in that it is large in size and heavy in weight because of the use of the inertia piston which also complicates its construction.
In the course of operation the spring of the control valve weakens resulting in delayed closing of the passage of the hydraulic distributor which is connected to the pressure line. This arrangement of the hydraulic distributor fails to provide high frequency percussions due to the delayed action of the control valve and limited spring life.
Recoil reaction in heretofore described device is reduced through the use of the inertia piston moving in a direction opposite to the movement of the piston hammer. The recoil reaction from the piston hammer and the inertia piston is directed opposite to their movement, the resulting force acting on the housing being equal to the algebraic sum, whereby the force of the recoil reaction is reduced. However, the inertia piston provided in the housing to serve as a means for reducing the recoil reaction fails to effect a useful function.
The resulting value of the recoil reaction is determined by the forces acting in the chambers of the pneumatic accumulators and the chambers connected to the pressure and discharge lines caused by the pressure of the compressed gas and that of the working fluid whose respective values tend to vary within a wide range thereby leading to alternating recoil reaction.
There is further known a hydraulic percussive tool (cf. USSR Inventor's Certificate No. 761,652, IPC E01C 19/30, E02D 3/046, published 1975) comprising pressure and discharge hydraulic lines, a housing having a piston hammer secured in guiding sleeves separating the interior of the housing into chambers connected by passages to the pressure and discharge hydraulic lines, and a chamber serving as a pneumatic accumulator. Attached to the housing is a hydraulic distributor the cylindrical interior of which accommodates a spring-loaded control valve separating it into two portions one of which communicates continuously by way of one of the passages of the hydraulic distributor with the chamber of the housing connected to the discharge hydraulic line and by way of another passage periodically blocked by the control valve is connected to the chamber of the housing communicating with the pressure hydraulic line.
A disadvantage of the above device resides in that the control valve spring weakens in the course of operation, which results in delayed closing of the overflow passage by the control valve and, consequently, in reduced frequency of percussions, impact power and the efficiency of the percussive tool. Therefore, the above construction fails to provide high frequency percussions because of the relatively delayed action of the control valve and limited service life of the valve spring.
In addition, acceleration of the piston hammer causes a recoil reaction acting upon the housing which is either hand-operated or mounted on a machine. The value of the recoil reaction is determined by the active area of the piston hammer and that of the housing on the side of the pneumatic accumulator. The recoil reaction acts to limit the impact power of the piston hammer thereby reducing impact strength and efficiency of the device. Also, the recoil reaction alternates because the piston hammer reciprocates relative to the stationary housing.
It is therefore an object of this invention to make the recoil reaction of a hydropneumatic percussive tool consistant in direction and minimal in value.
Another object is to improve the efficiency of the hydropneumatic percussive tool.
Still another object is to improve the operational stability of a hydraulic distributor of the hydropneumatic percussive tool and improve the reliability thereof.
One more object is to provide a hydropneumatic percussive tool of reduced weight and size.